Radiant Heating Trivia

There is some debate about when and where radiant heating was first developed. Traditional thought credits the Romans in the first century B.C. as the first to use the technology on a large scale. More recent discoveries indicate that various forms of the
technology existed in Korea long before the Romans began walking around on warm floors. Either way, these early radiant systems shared a common design: the hypocaust.

Hypocaust, from the Latin hypocaustum, translates as “fire beneath” or “heat from below.” The Romans were masters of designing hypocausts and used them in every facet of their lives, from public baths to personal residences.

But what is a hypocaust, really? The design of a hypocaust is fairly basic, as shown in following figure. The interior space is built on a series of stone pillars, each approximately two to three feet high and two feet square. The pillars are spaced far enough apart
to provide structural support, but not too close to hinder air flow.

The heart of the hypocaust consists of a large furnace, or fire pit, located just outside the main building and connected by a tunnel directly to the sub-chamber beneath the floor. Flues are integrated into the walls, causing a natural draft effect. Heat from the furnace is pulled by convection through the sub-chamber and then up the walls via the flues.
This process heats the floors as well as the walls, totally encapsulating the room with radiant surfaces.

To prevent overheated conditions, the structures were designed so that the areas requiring the most heat were located closest to the furnace. In most cases this was the bath itself, or some other water-based feature. The exhaust temperature would drop as it moved through the sub-chamber, allowing more comfortable floor temperatures in less active areas farther away from the furnace.

The use of the hypocaust faded into the history books along with the Romans, but fortunately the concept of warm floors did not. Today, radiant systems have been adapted to fit modern construction profiles.

The previous material was an excerpt from “Radiant Heating, Part 1: The Basics” by Kolyn Marshall, Director of Strategic Marketing, Watts Radiant, available as part of the “Service Application Manual” of the Refrigeration Service Engineers Society.

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